1. Field of the Invention:
The present invention relates to a novel sodium tripolyphosphate, a process for the preparation thereof, and to the use of same in various comestibles and foodstuffs, and in detergent compositions.
2. Description of the Prior Art:
Sodium tripolyphosphate is known to this art and is said to have the general formula Na.sub.5 P.sub.3 O.sub.10, but the apparent simplicity of the formula in reality disguises truly complex inorganic chemistry.
In theory, the tripolyphosphate is obtained from a solution of mono- and disodium orthophosphates by adjusting the Na/P ratio thereof to the approximate value of 1.667, in a process which can be represented as follows: ##STR1##
In reality, however, there exist an extensive number of possible reactions having several equilibria, which are summarized below (nonetheless, it is not claimed that each such reaction indeed occurs, to any extent whatsoever, or that the following list is exhaustive): ##STR2##
More simply, it will be appreciated that short chain (soluble or long chain (insoluble) sodium polyphosphates and sodium pyrophosphates can be obtained in addition to the desired tripolyphosphate.
The starting solution is readily prepared by neutralizing H.sub.3 PO.sub.4 with NaOH or by mixing NaH.sub.2 PO.sub.4 +2Na.sub.2 HPO.sub.4 orthophosphates. A heat treatment is then conducted upon the solution thus prepared by known processes, such as those involving spraying, rotary dryers, fluidized bed or open flame, in a single stage (or in two stages, if the intermediate orthophosphate is separated). Depending upon the particular thermocondensation conditions (temperature, partial pressure of water, Na/P ratio, impurities) the TPP obtained will be 100% phase I, 100% phase II or a mixture of phases I and II.
It too is known that in certain applications, particularly in the field of foodstuffs and other comestibles, and in detergent compositions, that if the TPP is dissolved in a medium which is either not agitated (quiescent) or inadequately (slightly) agitated, this typically produces lumps which are difficult to dissolve or result in caking or hardening. Thus, anhydrous phases I and II result in partial caking or hardening.
Hydrated phase II results in virtually instantaneous total caking or hardening, whereas hydrated phase I does not result in any caking or hardening. A TPP which will readily dissolve is thus one which contains less than 40% of phase I and which has been "prehydrated" (as in the test described below).
Prehydration can be effected by spraying water onto the TPP upon the synthesis thereof, or by the natural fixing of atmospheric-borne water. The amount of prehydration required to enable the TPP to be dissolved without solidifying in a non-agitated or quiescent medium is at least 0.6%, measured by weight loss at 150.degree. C.